Day Three: Wonky Worldview Shift (or) What's a Native Plant Anyway?

Today's field trip to the wooded lot across the street from the Arnold Arboretum left me sputtering. Peter Del Tredici, our guest speaker who led the trip, is a Senior Researcher at the Arnold Arboretum and a professor at Harvard University. That said, first and foremost, I give the man credence. Although his ideas are not of the mainstream (at least in the river I’m in), I certainly wouldn’t consider him a quack in the world of ecologists. Here’s what he thinks (at least based on my interpretation):

There is nothing “natural” about an urban ecosystem. Urban ecosystems have many things in common. They:

• Are subject to a great deal of abuse and disturbance from construction and human manipulation. This makes them, in many ways, similar to an early successional environment (which is, by definition, an area which has recently undergone a severe disturbance). Essentially, since the disturbance never goes away in the city, cities are stuck at that stage of succession.
• Have poor soil that is mostly fill and construction debris…when they have soil at all are not paved over altogether. Soil is often polluted and compacted, resulting in little oxygen access at the roots.
• “Floodplain-ish.” Because of the impermeable surfaces and poor soil quality, water percolates very slowly. Water will often runoff before permeating the soil, or only reach the first few inches.
• Have varying temperatures (often significantly warmer than historic temperatures) due to paving and the urban heat island effect; temperatures may be very different near a parking lot versus 20 feet away under some trees.
  

So, even if a swamp or a prairie or a forest was the native ecosystem where a city is now located, can you really claim that the features of those historic ecosystems still ring true? The ecosystem features (like soil, temperature, precipitation, natural flora/fauna) that once drew a plant to that area are no longer present. Peter doesn’t think that native vs. invasive plant discussions have a place in the city because there ARE NO PLANTS NATIVE TO A CITY. You have plants that are pre-adapted to survive in a city (which are often early successional plants…and invasive). And, the “invasive” plants didn’t cause they degradation to the ecosystem – they are a symptom of that degradation. So, if you don’t have native plants to displace, how do you have “invasives”? According to Peter Del Tredici, the native/invasive argument is really a values statement and, in an urban setting, is a waste of time.

WHOA. Talk about a difference in worldviews. That is kind of a slap in the face to all of the landscape coordinators and volunteers I know who are working (within city limits) on restoration ecology. I wonder (though I didn’t get a chance to ask) if Peter Del Tredici even believes in restoration or conservation ecology as valuable fields of work and study. And, if what he’s saying is true, should the Chicago Department of Environment encourage residents to plant native plants at their homes? Should Permitting and Enforcement be regulating potential terrestrial invasives in the city limits? Following Peter’s train of thought, ecologists and landscapers should be only be concerned with what grows well in the environment that we’ve been handed and what provides the beauty and the ecosystem services that we’re after.

Hmm. Remembering that Peter is not a quack (and understanding some of the arguments that he’s making as quite valid) I tried REALLY hard to keep an open mind, and to ask as many questions as I could throughout the course of our conversation. My thought was, “OK, if I buy into what this guy says and go back to Chicago and try to sell his points, what are the arguments I’m going to hear?” Unfortunately, I think I irritated the heck out of the poor man and my classmates…but we’ll all get over it because I do think it was worthwhile. At first, I kind of felt like he was giving us a canned spiel and that we should all just believe him because he said so. And I feel like many of the answers he gave to complex questions were flippant. Some of my major food for thought was:

• What about the borders between cities, suburban areas, rural areas, and natural areas. If we want to preserve our native spaces, how do you keep invasives OUT of those places? Couldn’t invasive species pose a risk to native species we WANT to preserve. Peter’s response: “There is no documented evidence of a plant extinction due to an invasive.” Um…OK. But, haven’t there been documented extirpations…and if enough extirpations happen…
  
  
• After a little more prodding, he encouraged me to “think of the intention of a space.” So, on a roadway, do invasives matter? Is sending crews to pull purple loosestrife along a highway a worthwhile endeavor? If we need to pull invasives, we should be targeting the natural areas that we want to protect. My qualm with this is that those highways then serve as corridors for the spread of that plant…which means you are essentially BRINGING invasive plants to the backdoor of areas we want to preserve. Are we just supposed to wait until those protected areas are in extreme risk? Rather than taking preventative measures, is he encouraging a model where we’re constantly on the defensive with an extreme sense of urgency?
  
  
• Part of the reason we tell folks to plant natives is because plants have evolved in that environment and have natural resistance to pests, are accustomed to the regional rainfall, prevent erosion, etc.
  
  
• My last question before I gave up was, “OK. So, another reason to encourage native vegetation is because it provides habitat for native wildlife. Do ‘urban wild plants’ (a.k.a. invasives) provide adequate replacements for that habitat? His response (greatly paraphrased): “Whatever.” Needless to say, Peter has a slightly anthropocentric worldview.
  

Quotable quotes:

With regards to the presence of invasive plants in Massachusetts:
“That can’t be changed.”
(His worldview might be a bit defeatist as well. Or maybe I am just dangling by the threads of my youthful naïveté.)

His key question regarding the evaluation of an urban ecosystem:
“Can it function as a native ecosystem should?”
(This is the part that really made me think. Why haven’t I considered that question before?!)

Key questions:

• Is there evidence of extirpation due to invasives? I don’t believe him.
• Do invasive plants decrease biodiversity?
• How do invasive species impact small world networks; do they pose a risk to keystone species?
• Who is currently asking these questions in an urban setting? And is there a school of thought with an answer?
• What about the other factors that contribute to a native plant’s range (which can often be very large), like temperature, day length, and other features dependent on seasonality or latitude – are those features significant enough to make a native plant “native,” even in a city?
• WHAT ABOUT WILDLIFE? What if you took Peter’s whole argument and reconsidered it with regards to native wildlife? Do the same points still hold? Wildlife is also ecosystem-dependent; they either cope, move, or go extinct, much like plants, in an urban ecosystem. So, what do people argue about native/invasive critters in cities?
• What are other common characteristics of a city? In hearing people talk about Boston and drawing comparisons to Chicago, I wonder if cities from the START were generally places that were “unproductive” land (i.e., not suitable for agriculture), so they became built environments. There’s a significant wetland history at least in these two cities…I wonder about other major cities.